The present invention generally relates to the control of the air/fuel mixture for small gasoline engines that utilize a carburetor, or some other means of providing fuel into the air stream using a venturi. More particularly, the invention relates to an apparatus and method for regulating the flow of fuel into the air stream using a fast acting solenoid. By controlling the flow of fuel into the air stream, the air-to-fuel (A/F) ratio can be controlled to ensure that the engine operates efficiently under varying loads.
It is well known that the A/F ratio for a typical low revolutions per minute (RPM) air cooled engine used on walk behind mowers, riding mowers and other equipment can be controlled by a carburetor. When the operator increases the RPM with a RPM-demand lever, the engine's throttle plate adjusts to meet the RPM demand and normally a RPM governing system continuously adjusts the throttle plate to meet the set RPM regardless of engine load.
Since engines of this type normally are air cooled, the A/F ratio is configured to get maximum power output without overheating the engine at maximum load. As such, the carburetor calibration normally is such that access to fuel is provided for assisting the cooling of the engine. Since carburetors typically used in these types of applications are of a fairly simple design, features that can enrich the fuel are not present. Due to this and other factors, the same A/F ratio that the engine requires operating at full load will often be supplied to the combustion chamber even when operating at lesser loads. When engines operate at lesser loads, the cooling effect from the fuel is not required; the result is an air-to-fuel mixture that is “rich”—or contains more fuel than needed—for partial load operations.
While using a rich mixture protects against overheating and assists the engine in reacting quickly to increased load demands, it also increases emissions and fuel consumption. With most engines of this type being operated predominantly at partial load, the A/F mixture does not need to be as rich. Thus the emissions and fuel consumption from engines operating at partial load are higher than if the A/F ratio could be adjusted leaner during partial load operation.
An additional problem can occur in small engines due to the lack of control over the flow of fuel into the combustion chamber of the engine. Many small engines are designed such that the exhaust valve remains open for a short time after the intake valve opens. Thus, for a brief period, unburned fuel can pass directly through the exhaust valve into the exhaust system. When fuel passes through the engine without contributing to combustion, the engine is not using fuel efficiently, and emissions will be increased.
It is well known that currently existing technology provides two main techniques for controlling the A/F ratio. One technique is through Electronic Fuel Injection (EFI) that can control A/F ratio cycle-to-cycle. EFI systems, however, are costly to implement due to the high complexity compared to a carburetor. The other common technique used to control the A/F ratio is by using an “air bleed system” that indirectly controls the fuel flow to the air stream. This system, however, usually cannot adjust on a cycle-to-cycle basis and also has longer delay times associated with the physics involved with the mixture adjustment.